The Economist explains

How does antimatter differ from matter?

ANTIMATTER is that rare thing in fundamental physics: an abstruse concept embraced by popular culture. That is because, as any fan of "Star Trek" or Dan Brown will tell you, when matter and antimatter collide, the two annihilate to produce pure energy—which makes for a splendid plot device for authors in need of nifty fuel or high explosives. More than the pyrotechnics, though, scientists are interested in comparing how antimatter and the common-or-garden variety behave in different situations, such as, most recently, in the presence of gravity. Discrepancies, they think, may shed light on why there is any matter in the universe at all. What have they found out so far?

Antimatter first popped out of an equation that Paul Dirac, a British theorist, was working on in 1928 to explain the behaviour of electrons. In order for the maths to work, Dirac found, the electron needed a counterpart with equal mass but positive, rather than negative, electrical charge. The positron, as the particle came to be known, was observed four years later by Carl Anderson, an American physicist studying cosmic rays. Positrons emitted by earthly radioactive sources such as isotopes of sodium are now used routinely in many areas (the familiar hospital PET scan stands for "positron-emission tomography").

Mathematically, particles and their anti-versions actually differ in two ways. Besides having opposite electrical charges, they also carry opposite values of a property called angular momentum, or spin: they are, again mathematically speaking, each other’s mirror images, albeit not in a standard two-dimensional mirror. (Which is why Dirac's equation actually yields four solutions, one for each possible combination of spin and charge.) As a consequence, matter-antimatter pairs can disappear in a puff of energy without breaking conservation laws which physicists regard as inviolate; the opposite values simply cancel each other out without producing a surplus of either charge or angular momentum. Some particles, like photons of light, carry no electric charge but can still have opposite spins. They are, in effect, their own antiparticles.

But the maths is not complete. If it were, equal amounts of matter and antimatter would have been produced in the big bang, only to annihilate each other immediately and produce a lifeless sea of photons rather than a universe of stars, planets, and scientists to ponder such conundrums. Sure enough, in 1964 some particles called kaons were shown not to respect the symmetry of charge and spin—or, to give its proper name, charge-conjugation/parity (CP) symmetry. Since then, hints of similar "CP violation" have been spotted in a number of other subatomic species. The latest such findings, concerning particles called strange B mesons, were presented on April 24th by LHCb, an experiment at CERN, Europe's main particle-physics laboratory. When all known differences are totted up, however, that still is not enough to account for the extent of ordinary matter's cosmic preponderance. Nor does it pin down what other, possibly as-yet unidentified symmetry that particles and antiparticles flout. (The latest results for their interaction with gravity are inconclusive.) So expect to hear plenty more in the coming years about antimatter, both in science fiction and science proper.

It would appear that every elementary particle in the Universe has a partner particle, known as an ‘antiparticle’. These shares many similar characteristics, but many other properties are the opposite of those for the particle. The electron, for example, has as its antiparticle the antielectron. The electron and the antielectron have congruous masses, but they have exactly opposite electrical charges.

The generally held perception is that the common stuff around us appears to be ‘matter’, but we routinely produce antimatter in small quantities in high energy accelerator experiments. When a matter particle meets its antimatter particle they destroy each other completely (i.e. annihilation), releasing the equivalent of their rest masses in the form of pure energy (according to Einstein’s Theory of E=mc²). For instance, when an electron meets an antielectron, the two annihilate and produce a burst of light which produces a corresponding energy level equivalent to the masses of the two particles.

Yet, because the properties of matter and antimatter mirror each other, scientists assume that the physics and chemistry of a galaxy made entirely from antimatter would closely parallel that of our matter galaxy. It is conceivable, therefore, that life built on antimatter could have evolved at other places in the Universe, just as life based on matter has evolved here.

However, we have no evidence of large concentrations of antimatter anywhere in the Universe. Everything that we see so far seems to be matter. And if true this is also something of an anomaly, because naively there are reasons from fundamental physics to believe that the Universe should have produced about as much matter as antimatter.

Dark matter, whilst it cannot be seen (even with our telescopes), is present because we see its gravitational influence on the rest of the Universe. Different experiments indicate that there is probably 10 times more matter in the Universe than the matter that we see. Essentially, dark matter is what the Universe is made out of, but we don’t yet know what it is. To illustrate the point of evidential dark matter, the velocity of rotation for spiral galaxies depends on the amount of mass contained in them. But the outer parts of our own spiral galaxy, the Milky Way, are rotating much too fast to be consistent with the amount of matter that we can detect.

There are some fairly strong arguments based on the production of the light elements in the Big Bang that suggest the majority of the dark matter cannot be ordinary matter or antimatter. Physicists refer to this as ‘baryonic matter’. This implies that the majority of the mass of the Universe is in a form very different from the matter that makes up the world around us.

Some scientists have hypothesised that dark matter (or at least part of it) could be antimatter, but there are very strong experimental reasons to doubt this. If the dark matter out there were antimatter, wouldn’t we expect it to annihilate with matter whenever it meets up with it, releasing bursts of energy primarily in the form of light?

By explaining more, this is more comprehensible to the layman than the blog post, but if you're going to get into conservation laws, energy and linear momentum are every bit as important as charge and angular momentum. Best probably to keep it to charge [just about everyone has heard of plus and minus charges] and energy. Angular and linear momentum are probably meaningless to most TE readers.
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As for spin, both electron and positron are spin 1/2, and opposite by convention, but the spins of the electron and the positron can be either parallel or opposite when annihilation occurs. The photons' spins take care of the conservation of angular momentum. So, while the net charge is zero, the net angular momentum need not, as you state above, be zero.

Thanks for this helpful article and the excellent comments too. "Anti-matter" seems a bit of a misnomer. In the ordinary meaning of the word, these particles are surely all matter, as is "baryonic matter". If there is baryonic matter, the seeming binary opposition of "matter" to "anti-matter" seems inappropriate. But this is just a semantic quibble from a non-scientist.

Hi,
An excess of antimatter within the cosmic-ray flux presents some problems. The origin of the excess, however, remains unexplained. An answer brings back supersymmetry, a theory thrown out in that positrons could be produced when two particles of dark matter collide and annihilate. In addition the binding energies of nucleons in the nucleus are several orders of magnitude smaller than the momentum transfers of deep-inelastic scattering, so, naively, such a ratio should be unity except for small corrections for the Fermi motion of nucleons in the nucleus, and it’s not why.

A slip of the keyboard. "create matter and antimatter" is, I'm sure, what you meant to type, because you obviously know what you're talking about.
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It might have been useful to specify that they are created together simultaneously out of the vacuum. And you, I think, thought it too obvious to say explicitly that the matter and antimatter annihilate within the period of uncertainty afforded by the uncertainty of the value of energy.
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But it might confuse someone who isn't familiar with this stuff.

Just to encourage you to stay away from elementary particle angular momentum, look up "positronium". The electron and positron enter a mutual orbital state before annihilating, and that makes it all that much more complicated. Of course, positronium doesn't belong in a simple exposition. But I also question whether particle spin belongs there, unless whoever is writing understands the extent to which simple analogies are appropriate, and can get them right.

I'm a layman, and this article helped me not. I'm genuinely curious to understand what anti-matter represents. If you manage to come up with some intuitive examples of what it is, such as representation that a kid would be able to imagine, it might help. But talking about CP violation et al. is likely to scare away all non-boffins.

What you are suggesting is that there is another region, completely empty (perfect vacuum).
The problem is, this does not exist. The vacuum fluctuates:
It is able to create matter "out of thin air" thanks to the energy-time uncertainty.
It is quite complicated physics but the basic principle is that energy can be "borrowed" for a very small time (thanks to the energy-time uncertainty principle) to form particles (E=mc^2). As a consequence, the vacuum is never really empty.
If there were three regions in our universe namely matter-preponderant, complete vacuum and antimatter-prepoderant we would still be able to observe evidence of matter-antimatter annihilation.

It is highly unlikely for the following reason:
Assuming the universe is "split in two": our region where matter is preponderant and another region where antimatter is.
There would be a frontier between these two regions where matter and antimatter would collide, and as a consequence annihilate and emit photons.
However there is no evidence of such a frontier, there is no source of photons characteristic of a massive matter-antimatter annihilation region.

Good job - you left out the unnecessary complications that TE added, and that they didn't understand.
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Sometimes we get so overwhelmed by what we know, that we forget just how deeply ignorant we are. I can't help feeling that dark energy and dark matter are analogous to the luminiferous ether. A change in perspective was all it took to banish it from physics.
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In special relativity, Einstein rejected the attempt to force the spacetime of Maxwell's equations of electromagnetism into the universally accepted Galilean/Newtonian space and time of classical mechanics. That's what the lumeniferous ether was supposed to do. Instead, Einstein asserted that Maxwell's spacetime described the real world, and that classical space and time were adequate approximations so long as speeds were well below the speed of light. Poof! and the luminiferous ether was out of a job.
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Maybe we're just not looking at things from the proper point of view.

Before all you particle nerds get too carried away, try this on for (brain) size: the "Theory of Absolute Relativity". Invented by a couple of acquaintances, it maintains that if you recalculate the constants just right, the theories of the very small and the very large can be reconciled. At last. Einstein meets Newton. http://arxiv.org/abs/0908.2562
Next question: how do we harness all those neutrinos to produce an infinite source of energy?
Paradox: anti-matter doesn't matter in the course of a normal lifespan!